Congo red and trypan red inhibited Newcastle disease virus neuraminidase totally and fowl plague virus neuraminidase partially in vitro, probably by polyanionic effects. The inhibitors did not prevent viral growth when present during the adsorption period. Congo red inhibited virus release and disturbed the synthesis of haemagglutinin, but did not interfere with the formation of ribonucleoprotein antigens. Trypan red, however, had no influence on the multiplication of either virus.